1. Field of the Invention
This invention relates to rare earth metal-transition metal-boron permanent magnets, and more particularly to a rare earth metal-transition metal-boron permanent magnet having improved corrosion resistance.
2. Description of the Related Art
The use of rare earth metal magnets of high energy product type has rapidly increased recently in accordance with the miniaturization and high efficiency of electronic parts. Among them, Nd--Fe--B type magnets are particularly preferential in place of Sm--Co type magnets. The Nd--Fe--B magnets are advantageous in resource and also exhibit excellent magnetic properties capable of attaining a high energy product as compared with the Sm--Co type magnet (as described in JP-B-61-34242).
In the Nd--Fe--B type magnet, however, neodymium as a light rare earth metal and iron are used as main components, so that the corrosion resistance is poor. That is, this magnet produces rust with the lapse of time even in a normal atmosphere. Such an occurrence of rust considerably degrades the reliability of the magnet, which obstructs the application of the magnet in wider environments.
In this connection, the inventors have proposed alloys obtained by compositely substituting a part of Fe with Co and Ni in JP-A-2-4939 and magnets prepared by mixing RE.sub.2 TM.sub.14 B with RE-TM alloy in JP-A-3-250607 as a means for improving the corrosion resistance.